Adjustable center loaded antenna arrangement

ABSTRACT

A center-loaded antenna arrangement having a fixed loading coil in a whip antenna, and a tuning core mounted for movement through and out of the coil, said core having a ferrous section and an inductor ring section. Suitable connecting means connects an end of the coil to a radio ground and to a radio transmitter or receiver through a R.F. transmission line. This connecting means is preferably connected to the radio ground through an impedance matching device which couples the coil to the transmitter or receiver.

United States Patent SpiIsbury et al.

[ 1 June 20, 1972 [54] ADJUSTABLE CENTER LOADED ANTENNA ARRANGEMENT [72]Inventors: Ashton James Spilsbury, 6691 Madrona Crescent, WestVancouver, British Columbia; Oswald Thorkelson, 3182 To]- mie Street,Vancouver, British Columbia,

both of Canada [22] Filed: Jan. 7, 1970 {21] App1.No.: 1,141

521 u.s. CI .343/750, 343/703, 343/715,

3,226,725 12/ 1965 Ritchie et a1 ..343/ 750 3,381,222 4/1968 Gray343/750 2,541,107 2/1951 Selgin..... ..343/861 2,920,323 l/1'960 Dunson..343/850 3,160,832 12/1964 Beitman et al. 343/861 3,513,472 5/1970Altmayer ..343/750 3,540,057 11/1970 Piersson et a1 ..343/750 PrimaryEvanu'ner-Eli Lieberman Att0meyFetherstonhaugh & Co.

[57] ABSTRACT A center-loaded antenna arrangement having a fixed loadingcoil in a whip antenna, and a tuning core mounted for movement throughand out of the coil, said core having a ferrous section and an inductorring section. Suitable connecting means connects an end of the coil to aradio ground and to a radio transmitter or receiver through a RF.transmission line. This connecting means is preferably connected to theradio ground through an impedance matching device which couples the coilto the transmitter or receiver.

9 Claims, 7 Drawing Figures TOP PORT/0N 3O mrseusomrs sin/0M 28 Bassmer/av 26 P'ATENTEnJum m2 3.671.972 sum 10F a TOP PORT/0N 30 INTERMEDMTE8465 mkr/av 26 1 40 "iv ENTO R5 ASHTON JAMES SPILSBURY lgfi THORKELSONOSWALD ATTORNEYS PATENTEDJUHZU I972 SHEET 2 [IF 3 \NVENTOR ASHTON JAMESSPILSBURY THORKELSON OSWALD ATTORNEY PATENTEnJum I972 3.671.972

sum 30F 3 INVENTORS A SHTON JAMES SPILSBURY THORKELSON OSWALD ATTOR NP/SADJUSTABLE CENTER LOADED ANTENNA ARRANGEMENT BACKGROUND OF THE INVENTIONThere is a great need for a center-loaded mobile antenna which is ofsmall dimensions, light in weight (particularly for use in aircraft),efficient over a large frequency range, permits coupling to atransmitter over a shielded, coaxial cable with very low voltagestanding wave ratio (VSWR), is tunable accurately from a remotelocation, is completely waterproof, and rugged enough to withstandreasonable abuse and high air speed. There is no antenna on the marketthat meets all of I these requirements.

One of the main difficulties in designing an antenna having thesefeatures is to achieve a constant base resistance over its entire tuningrange. This is necessary to avoid numerous matching changes between theantenna and the transmission line connected thereto to accommodatedifferent frequencies. It is also necessary to attain accurate tuningand matching of the antenna to avoid high standing waves on thetransmission line with the resultant losses. More important thanthelosses from standing waves, however, is the effect of reactivetransmission line loads on transmitters, particularly single sidebandtransmitters which employ linear amplifiers. This reactive load caneasily reduce the output power of a single sideband (SSB) transmitter by50 percent or more and introduce severe distortion in the linear poweramplifier.

The base resistance of a center-loaded whip antenna is dependent mainlyon two factors, (1) the total physical length of the antenna versus thefrequency of operation, and (2) the Q or quality factor of the centrallylocated loading coil.

SUMMARY OF THE INVENTION This invention relates to adjustablecenter-loaded antenna arrangements or systems particularly suitable forvehicular operation.

An adjustable center-loaded antenna arrangement according to the presentinvention comprises a bottom portion, a hollow intermediate portionconnected at one end to the base portion, and a top or outer portionconnected to the opposite end of the intermediate portion. A loadingcoil is fixedly mounted in the intermediate portion and has one end tobe connected to a radio frequency transmitter and an opposite endconnected to the top portion. A tuning core is mounted in theintermediate portion for movement through and out of the coil, this corehaving a high permeability ferrous section axially aligned with aninductor section.

The total physical length of the antenna is set by practicalconsideration, and around 9 feet has been found to be a practical lengthfor many purposes. The radiation resistance of an antenna of this lengthat a frequency of 2 MHz is approximately, 1.5 ohms, and at 8 MHz it isapproximately 3.5 ohms. The resistance of this whip antenna would be 1.5ohms, plus the resistance of the loading coil, at 2 MHz and 3.5 ohms,plus the resistance of the loading coil at 8 MHz.

The major portion of the resistance of the antenna is the loading coilresistance, while the radiation resistance is usually relatively small.By using the highest quality material available in the ferrous sectionof the core, an optimum wire size and turns spacing, the Q" of theloading coil is maintained at a high level at the lower frequencies, inwhich condition the ferrous section of the core is in the coil. Towardthe high frequency end of the tuning range, the inductor ring section ismoved into the coil and the ferrous section is moved out. At the highestfrequency all the ferrous section is out of the coil and the entireinductor ring section is in it. The inductor section is made up of aplurality of spaced copper inductor rings.

MHz, resulting in a base resistance centering at 32 ohms. The tuningcoil is connected by a coaxial cable to the transmitter. In thisexample, a fixed matching network is used between the 52 ohmtransmission line or cable and the base of the antenna to match downfrom 52 to 32 ohms. The VSWR on the 52 ohm transmission line with thisantenna properly installed is typically 1.121 or lower over the entirefrequency range of 2 to 8 MHz.

With this antenna, it is possible to attain a substantially constant RFantenna base resistance over a wide range of frequencies. This isaccomplished by the specific core construction and association with thetuning coil and the connection of the end of the coil to a radio groundthrough an impedance matching device for coupling it to a radiotransmitter or receiver through a RF transmission line. The coreincludes a ferrous section and an inductor ring section. A high Qmaterial is used in the ferrous section, the rings are made of highlyconductive material, and the length, thickness and spacing of said ringsare selected to produce the desired change in the inductance and Q ofthe tuning coil.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 diagrammatically illustrates anexample of antenna arrangement or system in accordance with thisinvention,

FIG. 2 is a diagrammatic view of the interior of the antenna of thissystem,

FIG. 3 is an enlarged sectional view through the loading coil and tuningcore of the antenna,

FIG. 4 is a wiring diagram of the antenna,

FIG. 5 is a wiring diagram of an example control unit in this system,

FIG. 6 is a wiring diagram of an example SWR detector used in thesystem, and

FIG. 7 is a view similar to FIG. 3, illustrating a preferred form ofloading coil and tuning core for the antenna.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Some values are set out in thewiring diagrams, but it is to be understood that these are for thepurpose of illustration only, and are not intended to be used in alimiting sense.

Referring to FIG. 1 of the drawings, this depicts an adjustablecenter-loaded antenna arrangement or system 10 in ac cordance with thisinvention, this system including an antenna 11, a control unit 12, and aVSWR sensor 13. The control unit is connected to a suitable source ofpower, not shown, by wires 16, and to antenna 11 by a multiple wirecable 18. This control unit is also connected to the sensor unit 13 bywires 20. The SWR sensor unit is connected by a coaxial cable 22 to aradio transmitter or receiver, not shown, and by a coaxial cable 23 toantenna 11.

The antenna 11 includes a bottom portion 26, usually made of aluminum,an intermediate section 28, preferably made of fiber glass or any othersuitable non-conducting material, and an outer or top portion, 30usually made of a flexible wire enclosed in fiber glass. The antennaportions 26, 28 and 30 are permanently or removably connected to eachother in any desired manner. The terms bottom" and top" are used fordescriptive purposes only since the antenna functions in any position,such as vertical, horizontal, upside down, and the like.

Referring to FIG. 2, a center-loading coil 34 is fixably mounted withinintermediate portion 28 and is connected at one end by means of springcontact 35 to top portion 30, and at the other end by a wire 36 to thecenter conductor of coaxial transmission line 23. One side of animpedance matching device, such as a capacitor 37, is connected by saidwire 36 to coil 34, and the other side of said capacitor is connected bya wire 38 to the outer shielding of the line 23. At the same time, wire38 is connected by means of spring contact 39 to the inside of thealuminum bottom portion 26. This bottom portion is connected to theradio ground, as indicated at 40 in FIG. 2.

A tuning core 41 is mounted for movement into and through coil 34, andthis core is made up of a ferrous section 42 and'an inductor ringsection 43. The ferrous and ring sections 42 and 43 are of such lengthand are positioned relative to each other so that when one section iscompletely within the coil, the other section is completely outside it,and yet portions of both sections can be in the coil at the same time.For example, the section lengths may be such that both will fit withinthe coil at the same time, or each section may be substantially the samelength as the coil. FIG. 3 shows the ferrous section 42 and the inductorring section 43 of such lengths that both can fit in coil 34 at the sametime, whereas FIG. 7 shows a ferrous section 42a and a ring section 43a,each of which is substantially the same length as coil 34.

Power means is provided for moving core 41 relative to coil 34. In thisexample, there is mounted in bottom portion 26 of the antenna areversible electric motor 45 which turns through gears 46 a worm orscrew 47 upon which a nut 48 is threaded. A stiff connector or rod 49extends between and is connected to core 41 and nut 48. Limit switches51 and 52 are provided at the ends of the path of travel of nut 48 so asto be engaged by the latter in order to prevent core 41 from being movedtoo far in either direction relative to the loading coil. Suitable meansis provided for indicating the linear position of the tuning core, andthis may be in the form of a sliding resistance wire bridge having aslide 56 and mounted in base portion 26, said slide being connected toand moved by nut 48. The resistance of the bridge is proportional to thelinear position of the tuning core and indicated by a meter on thecontrol panel.

FIG. 3 illustrates coil 34 and core 41. Coil 34 is made up of a wire 58space wound on a thinwall fiberglass tubing 59. The wire is embedded ina resinous coating 60 which may be an epoxy resin. The ferrous section42 of the core is made of short cylindrical ferrous slugs 62 which areresiliently held together under tension by rod 64 formed of anon-conducting material, such as teilon. Although the ferrous sectionmay be in the form of a single ferrous slug, it is preferably made up ofa plurality of slugs, as shown. The slugs are made of compressedpowdered iron. The material of the slugs is one displaying the highestpermeability with the lowest losses over the frequency range for whichthe antenna is designed. Rod 64 extends from ferrous section 42 toinductor ring section 43. This section 42 consists of copper conductorrings 66 mounted on rod 64 and separated from each other bynonconducting spacers 68 to form gaps 69 therebetween. These gaps are ofsufficient width to prevent short circuiting between adjacent rings.Each conductor ring 66 is preferably in the form of a cup having a base70 through which rod 64 slidably extends. Each spacer 68 is a sleevefitting within the ring or cup 66 and bearing at one end against cupbase 70, and at its opposite end projecting beyond the ring, asindicated at 71, to bear against the outer surface of base 70 of thenext adjacent base, as clearly shown in FIG. 3. The complete coreconsists of coupling 73 to which rod 64 is attached, a non-conductingspacer 74 between said coupling and the end slug 62, ferrous slugs 62,spacers 68, and copper inductor rings 66. The right hand slug 62 bearsagainst the end of the adjacent spacer sleeve 68, and the right handring or cup 66 bears against a non-conducting stop or disc 76 which iscarried by rod 64 and is held in place by a nut 77 threaded on the endof said rod. A set screw 79 in coupling 73 secures core 41 to rod 49.

The size and spacing of the turns of wire 58 of coil 34 are determinedto provide the correct inductance for the frequency range desired andalso with regard to the amount of RF power for which the antenna isdesigned. As one example of an antenna designed to cover 2-8 MHz andwith a maximum power rating of 120 watts peak power, l96 turns of No. 22B & S gauge wire wound on a 13/16 inch round coil form and evenly spacedto a total length of 7 inches have been used. These dimensions areillustrative only, and the dimensions must be established on a case bycase basis.

The optimum number, diameter and wall thickness of the inductor ringsare dependent on several factors, the major ones being:

a. Maximum Tuning Effect The inductor ring operates in accordance withLenz Law which shows that the current induced in the inductor ring hasphase difference to the current in the coil. The flux in the inductorring opposes the flux in the main coil and reduces the total inductanceof the coil. This has the effect of increasing the resonant frequency ofthe antenna. In order to attain the maximum effect it is necessary thatthe outer diameter of the inductor ring closely approximates the innerdiameter of the coil. In other words, the space between the inductorring and the coil must be kept to a minimum consistent with providingthe required insulation between the two.

b. Minimum Electrical Losses in the Inductor Rings If the inductor ringextends over a considerable part of the total length of the coil acapacitively coupled circuit is presented across the axial length ofthis portion of the coil and across the high R.F. voltage existingtherein. This results in an unwanted and wasteful current being set upthrough the length of the inductor ring which lowers the voltage acrossthe coil and reduces its effectiveness. In order to minimize this lossit is necessary to divide the inductor ring into a number of sections.The number of divisions used is limited only by the added complexity andcost of the design. In practise, it has been found that an acceptablecompromise is to reduce the length of the individual inductor ringsection to a figure not exceeding its diameter.

c. Minimum Capacitive Coupling Between Sections In order to furtherreduce the loss current set up through the series of inductor rings itis desirable to reduce the wall thickness of the rings as much aspossible and thus reduce the electrical capacity between rings. Thepoint beyond which the thickness cannot be effectively reduced isreached when the current carrying capacity of the ring becomesinsufficient and/or the mechanical strength of the ring is insufficient.As an example a wall thickness of 0.001 in. has been found to besatisfactory in this antenna.

d. Mechanical Flexibility In order to sustain bending and impact strainsin mobile operation it has proved advantageous to divide the length ofthe inductor ring into many sections flexibly attached to each other.This is accomplished automatically when the conditions outlined in a, b,and c above are complied with.

When ferrous section 42 of tuning core 41 is moved into coil 34, it hasthe effect of increasing the inductance of the coil and lowering thefrequency. When inductor section 43 is moved into the coil, theinductance is decreased and the frequency is increased.

The intermediate portion 28 of the antenna has a certain amount offlexibility, and the making of the ferrous section 42 in a plurality offerrous slugs spring loaded together provides a 7 degree of flexibilityin this section and helps to avoid mechanical damage from impact andbending of the whole structure due to inertia of air drag loading.

Control unit 12 includes a tuning meter 81, a two-position controlswitch 82, and a motor control switch 83. When switch 82 is in oneposition, meter 81 is connected to the resistance bridge 55 of theantenna and indicates the linear position of the tuning core 41 relativeto coil 34. When this switch is in its other position, it connects themeter to the VSWR sensor unit 13 to indicate the ratio of reflectedpower to forward power in the coaxial transmission line 23, and is usedto indicate the exact adjustment at which the minimum reflected power isattained as a result of the tuning of the antenna.

The VSWR sensor unit, which is of a type well known in the industry,contains the necessary circuitry to detect forward or backward power inthe coaxial transmission line to provide an indication on the meter inthe control unit.

The coaxial transmission line 23 is terminated in antenna 11 in such away as to match the impedance of the antenna to that of the coaxialtransmission line. The values of capacity and inductance introduced tothe antenna circuit by means of the tuning element are criticallydesigned to provide an apparent constant electrical impedance at theantenna connection so that uniform power transfer between the coaxialtransmission line and the antenna through the coupling device isattained over the entire tuning range.

An amplified R/C' circuit, indicated at 85 in FIG. 5, is placed incircuit with motor 45 and switch 83 to provide a high startingresistance when the control switch is operated. This results in themotor starting at a slow speed and increasing gradually to full speed,thus facilitating fine positioning of the tuning core in the loadingcoil.

, The above system readily lends itself to full automatic tuning of theantenna. In this case, the sensor will detect whether the antennaappears inductively or capacitively reactive and thence throughappropriate electrical circuiting and switching devices will cause themotor to so adjust the position of the core relative to the tuning coilto correct the capacitive/inductive inbalance and thus tune the antennaautomatically to the frequency of the R.F. output of the transmitter.

Some of the advantages of the present center-loaded antenna arrangementcore systems are as follows:

1. By designing a tunable antenna with an integral constant coupling toa coaxial transmission line it is possible to avoid entirely the use ofany open connecting or lead wires carrying R.F. voltages or R.F. currentthus avoiding power loss in transmission and unwanted noise pick up inreception.

2. By means of the motorized control of the tuning of the antenna, theantenna can be located advantageously as far as transmission efficiencyis concerned, and practically without regard to the length or positionof the wires and transmission cable.

3. The extended connector 49 from the motor to the tuning element makesit possible to locate the tuning coil at the most efficient position inthe antenna to accomplish maximum radiation efficiency (center-loading),and at the same time permits keeping the electric motor and associatedwiring removed from the active radiating portion of the antenna whereits presence could introduce losses and interference.

4. By a careful choice of the ferrous material and the inductor rings ofthe tuning core it is possible to provide an antenna which hasrelatively high Q or high radiation efficiency over the entire tuning.

. By carefully designing and maintaining a correct proportion of theamount of inductance introduced by the ferrous core and the amount ofinductance cancellation provided by the inductor rings, it is possiblein this antenna to present a constant load impedance, for example, 32ohms, over the entire tuning range.

6. By careful designing and dimensioning of the inductor rings, it ispossible to attain maximum tuning effect with a minimum of electricalloss, which in all other cases results from the use of inductor rings.These rings are insulated one from the other, and in cross sectionpresent a minimum of capacitance from one ring to the other, thusminimizing the R.F. power loss that would otherwise occur.

7. The dividing of the ferrous section of the core into a plurality ofsections resiliently held together provides for flexibility and avoidsmechanical damage from impact or strain.

. The meter of the control unit when in the tune position indicates therelative linear mechanical position of the tuning core in the coil, andthe scale of the meter is graduated directly in frequency.

9. Because of the coaxial transmission cable and coupling device whichis located in the body of the antenna, the main base of the antenna isat ground potential and requires no insulation when mounted on the frameof a vehicle or craft.

10. By means of special circuitry which introduces a voltage drop in themotor circuit at the start of any adjustment, the speed of the motor isrelatively slow for the first second of duration, speeding upthereafter. This makes possible delicate adjustment of the position ofthe tuning element.

11. The electrical design of the antenna provides continuous tuningadjustment over the entire tuning range thereof without the use ofelectrical relays, tap switches or sliding contacts on the antenna coil,and other devices that normally introduce mechanical wear and faultyelectrical contact.

12. The design of the entire antenna unit is such that one can be madeweighing no more than five pounds and suitable for small aircrafi andhelicopters.

13. Since this antenna system eliminates the necessity for separatelyadjusting the impedance matching ratio with each change of frequency,and the tuning adjustment has been reduced to a simple linear mechanicalmovement, the arrangement is ideally adaptable to complete automaticoperation.

In approaching the problem of the design of a continuously tunablemobile antenna the designer has always been faced with the requirementto simultaneously control two separate variables, namely,

' l. Tuning the antenna to resonance mitter frequency.

2. Varying the ratio of the impedance matching device to comply withvarying base resistance of the antenna with each change of frequency.

These two adjustments are critical to the overall efficiency of theantenna system and must be synchronized.

in this invention the second adjustment has been entirely eliminated. Onexperimenting with the loading coil of the antenna, it was noted that asthe inductance of the coil was varied by use of inductor rings the Q wasalso affected. The more rings that were inserted into the coil the lowerthe inductance and also the lower the Q". These phenonema have been usedherein for a new purpose, that is, to maintain a constant baseresistance of the antenna over its frequency range in addition to tuningthe antenna.

Ferrous slugs have been incorporated to increase the tuning range andalso affect the above mentioned new purpose". in order to do this the Qof the center loading coil has to keep rising steadily as the ferrousslugs are inserted. This extended effect is accomplished by using high 0low permeability iron dust core slugs in conjunction with the inductorrings so that the resulting Q rises steadily from the high frequency tolow frequency end of the tuning range. The shape (length, thickness,spacing) of the rings, plus correct choice of available tuning slugmaterial results in a coil with the properties which most closelyresemble the ideal requirement.

It is believed that this is the first time the above method has beenused to achieve a near constant base resistance over a large tuningrange.

The following chart illustrates the theoretical development of centerloaded whip antenna for constant base resistance.

with the radio trans- Q=Quality factor of coil. XL=Inductivc reactanceof coil. R= Resistance of the coil (R.F.). BR =Basc resistance.

gR= Radiation resistance.

I. An adjustable center-loaded whip antenna arrangement comprising ahollow antenna; a loading coil fixedly mounted in the antenna; and atuning core mounted for movement through and out of the coil, said corecomprising means, comprising a high permeability ferrous section and aninductor ring section, for providing a decrease in the Q of the loadingcoil with an increase in the tuning frequency of the coil such that asubstantially constant radio frequency antenna base resistance isprovided over a wide frequency range; the inboardmost end of saidferrous section lying directly adjacent to the inboardmost end of saidinductor ring section and the spacing between the outer diameter of theinductor ring section and the inner diameter of the loading coil being aminimum consistent with the necessary insulation required between thecore and coil.

2. An antenna arrangement as claimed in claim 1 in which the inductorring section of the core is made up of a plurality of coaxial inductorrings spaced from each other sufficiently to prevent a short circuittherebetween.

can

3. An antenna arrangement as claimed in claim 1 in which the ferroussection of the core is made up of a plurality of ferrous slugsresiliently held together as a flexible unit.

4. An antenna arrangement as claimed in claim 1, in which the ferroussection and the ring section of the core are both of such length thateach may be in the coil alone.

5. An antenna arrangement as claimed in claim 1 in which the ferroussection and the ring section of the core are each substantially the samelength as the coil.

6. An antenna arrangement as claimed in claim 1 in which the inductorring section of the core is made up of a plurality of coaxial inductorrings spaced from each other sufliciently to prevent a short circuittherebetween; and said rings are not substantially longer than onediameter thereof.

7. An adjustable center-loaded antenna arrangement as claimed in claim 1in which the inductor ring section of the core is made up of a pluralityof coaxial inductor rings spaced from each other sufficiently to preventa short circuit therebetween, and the ferrous section is made up of aplurality of ferrous slugs, said inductor rings and said ferrous slugsbeing resiliently held together.

8. An adjustable center-loaded antenna arrangement as claimed in claim 1further comprising a coaxial R.F. transmission line for connecting anend of the coil to a radio frequency ground through an impedancematching device for coupling said coil to a radio transmitter orreceiver, said impedance matching device comprising a capacitorconnected at one side thereof to the center conductor of the coaxialline and to a conductor extending to the base of the coil, and at theother side thereof to ground.

9. An adjustable center-loaded antenna arrangement as claimed in claim 8in which said ground comprises a grounded base portion of the antenna,said other side of the capacitor being connected to the outer shieldingof said coaxial line.

1. An adjustable center-loaded whip antenna arrangement comprising ahollow antenna; a loading coil fixedly mounted in the antenna; and atuning core mounted for movement through and out of the coil, said corecomprising means, comprising a high permeability ferrous section and aninductor ring section, for providing a decrease in the Q of the loadingcoil with an increase in the tuning frequency of the coil such that asubstantially constant radio frequency antenna base resistance isprovided over a wide frequency range; the inboardmost end of saidferrous section lying directly adjacent to the inboardmost end of saidinductor ring section and the spacing between the outer diameter of theinductor ring section and the inner diameter of the loading coil being aminimum consistent with the necessary insulation required between thecore and coil.
 2. An antenna arrangement as claimed in claim 1 in whichthe inductor ring section of the core is made up of a plurality ofcoaxial inductor rings spaced from each other sufficiently to prevent ashort circuit therebetween.
 3. An antenna arrangement as claimed inclaim 1 in which the ferrous section of the core is made up of aplurality of ferrous slugs resiliently held together as a flexible unit.4. An antenna arrangement as claimed in claim 1, in which the ferroussection and the ring section of the core are both of such length thateach may be in the coil alone.
 5. An antenna arrangement as claimed inclaim 1 in which the ferrous section and the ring section of the coreare each substantially the same length as the coil.
 6. An antennaarrangement as claimed in claim 1 in which the inductor ring section ofthe core is made up of a plurality of coaxial inductor rings spaced fromeach other sufficiently to prevent a short circuit therebetween; andsaid rings are not substantially longer than one diameter thereof.
 7. Anadjustable center-loaded antenna arrangement as claimed in claim 1 inwhich the inductor ring section of the core is made up of a plurality ofcoaxial inductor rings spaced from each other sufficiently to prevent ashort circuit therebetween, and the ferrous section is made up of aplurality of ferrous slugs, said inductor rings and said ferrous slugsbeing resiliently held together.
 8. An adjustable center-loaded antennaarrangement as claimed in claim 1 further comprising a coaxial R.F.transmission line for connecting an end of the coil to a radio frequencyground through an impedance matching device for coupling said coil to aradio transmitter or receiver, said impedance matching device comprisinga capacitor connected at one side thereof to the center conductor of thecoaxial line and to a conductor extending to the base of the coil, andat the other side thereof to ground.
 9. An adjustable center-loadedantenna arrangement as claimed in claiM 8 in which said ground comprisesa grounded base portion of the antenna, said other side of the capacitorbeing connected to the outer shielding of said coaxial line.